The syndrome of X-linked mental retardation with fragile is the single most common form of inherited mental retardation. The fragile X syndrome is the result of amplification of a simple repeated DNA sequence CGG located within the first exon of FMR-1, a gene encoding an RNA-binding protein. In normal individuals, the first exon of FMR-1 contains between 2 and 60 copies, in tandem, of the trinucleotide repeat cytosine-guanine-guanine, (CGG), with an occasional TGG occurring once every 7-25 triplet repeat units, which is thought to stabilize the repeat. In some individuals, the number of trinucleotide repeat units is increased to between 60 and 200, making them carriers of a clinically silent permutation, which, however, can expand in their offspring to a full mutation containing many hundreds to thousands of copies of the repeat and leading to loss of FMR1 expression. We have cloned 86 copies of the CGG repeat, containing two TGG interruptions, into bacterial plasmids. The construct is unstable in bacteria but is stable in transgenic mice. We are introducing constructs containing CGG tandem repeats of varying lengths and containing varying numbers and location of TGG interruptions into transgenic mice to study when and how amplification of the permutation to full mutation occurs.